Measurement of the differential and integral distribution of diffuse x-ray scattering intensity from defects in thin strained layers

Double-and triple-crystal diffractometry have been used to study structural perfection of a ∼1 μm-thick Ga_1−x In_xSb_1−y As_y epitaxial film (x=0.9, y=0.8) on GaSb. It is shown that scattering from samples of this system can be divided into coherent and diffuse. The arrangement of reciprocal-lattice points of the film and substrate in the two-dimensional intensity distribution for asymmetrical reflections argues for the absence of elastic-strain relaxation. No dislocation networks are formed, and the diffuse scattering is produced by Coulomb-type defects. Localization of diffuse scattering in reciprocal space suggests that these defects reside in the epitaxial film. The diffuse-scattering distribution in asymmetrical reflections is shown to be anomalous; namely, it extends in a direction parallel to the surface and is split into two maxima. Schemes have been proposed and realized for measuring integral distributions of diffracted intensity along the surface and perpendicular to it, and their potential for studying diffuse scattering from defects is explored. Fiz. Tverd. Tela (St. Petersburg) 39, 1188–1193 (July 1997)     

Structure, electrical properties and temperature characteristics of?Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–Bi0.5Li0.5TiO3 lead-free piezoelectric ceramics

(1−x−y)Bi_0.5Na_0.5TiO₃–xBi_0.5K_0.5TiO₃– yBi_0.5Li_0.5TiO₃ lead-free piezoelectric ceramics have been prepared by an ordinary sintering technique, and their structure, electrical properties, and temperature characteristics have been studied systematically. The ceramics can be well-sintered at 1050–1150 °C. The increase in K⁺ concentration decreases the grain-growth rate and promotes the formation of grains with a cubic shape, while the addition of Li⁺ decreases greatly the sintering temperature and assists in the densification of BNT-based ceramics. The results of XRD diffraction show that K⁺ and Li⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure. As x increases from 0.05 to 0.50, the ceramics transform gradually from rhombohedral phase to tetragonal phase and consequently a morphotropic phase boundary (MPB) is formed at 0.15≤x≤0.25. The concentration y of Li⁺ has no obvious influence on the crystal structure of the ceramics. Compared with pure Bi_0.5Na_0.5TiO₃, the partial substitution of K⁺ and Li⁺ for Na⁺ lowers greatly the coercive field E _c and increases the remanent polarization P _r of the ceramics. Because of the MPB, lower E _c and large P _r, the piezoelectricity of the ceramics is improved significantly. For the ceramics with the compositions near the MPB (x=0.15–0.25 and y=0.05–0.10), the piezoelectric properties become optimum: piezoelectric coefficient d ₃₃=147–231 pC/N and planar electromechanical coupling factor k _P=20.2–41.0%. In addition, the ceramics exhibit relaxor characteristic, which probably results from the cation disordering in the 12-fold coordination sites. The depolarization temperature T _d shows a strong dependence on the concentration x of K⁺ and reaches the lowest values at the MPB. The temperature dependences of the ferroelectric and dielectric properties at high temperatures may imply that the ceramics may contain both the polar and non-polar regions at temperatures above T _d.     

Lpe growth method possibilities in A3B5 semiconductors preparation

Pre-diffusion limited technique of liquid phase epitaxy is used for preparation of quaternary solid solution Ga _x In_1−x As _y P_1−y and binary InP. Surface morphology of the layers prepared at this condition has been compared with those prepared by conventional LPE. Quaternary strained layers with composition near to Ga_0.21In_0.79As_0.75P_0.25 were grown with perpendicular lattice mismatch up to 1.6% in compression, on the InP substrates of (100) orientation. The used epitaxial technique has been modified for perspective attempt to heal the growth interfaces. Modification was tested by InP growth. Presented at the 6th Joint Seminar “Development of Materials Science in Research and Education”, Karlštejn, Czech Republic, 17–19 September 1996.     

Thermoelectric properties of n-type Sr x M y Co4Sb12 (M=Yb, Ba) double-filled skutterudites

Alkaline-earth (AE) and rare-earth (RE) atoms are usually used as void fillers in the caged compound CoSb₃ to improve the thermoelectric performance of the filled system. Polycrystalline single-filled Sr_0.21Co₄Sb₁₂, double-filled Sr _x Yb _y Co₄Sb₁₂, and Sr _x Ba _y Co₄Sb₁₂ skutterudites have been synthesized. Rietveld structure refinement confirms that both Sr and Yb occupy the Sb-icosaedron voids in skutterudite frame work. In this paper, we report the high-temperature thermoelectric properties including electrical conductivity, Seebeck coefficient, and thermal conductivity. Double filling of the Sr–Yb combinations shows a stronger suppression on lattice thermal conductivity than that of Sr–Ba combination. Furthermore, the double-filled Sr _x Yb _y Co₄Sb₁₂ skutterudites exhibit a much higher power factor than the Sr-filled system. The maximum power factor for Sr_0.22Yb_0.03Co₄Sb_12.12 reaches 41 μW cm⁻¹ K⁻² at room temperature and 57.5 μW cm⁻¹ K⁻² at 850 K, respectively. The enhanced thermoelectric figures of merit are 1.32 for Sr _x Yb _y Co₄Sb₁₂ and 1.22 for Sr _x Ba _y Co₄Sb₁₂ at 850 K, respectively.     

Absorption spectra and magneto-optic figures of merit in the Bi x Sm3-x Fe5-y Ga y O12 system

The absorption (α) and Faraday rotation (θ) spectra of 14 garnets belonging to the series Bi _x Sm_3-x Fe_5-y Ga _y O₁₂ (0<x<1.05, 0.8<y<1.15) have been measured between 15 000 cm⁻¹ and 19 000 cm⁻¹. The figure of merit (θ/α) at 17 850 cm⁻¹ (560 nm) increases linearly with increasing bismuth concentration up tox∼0.6 where it begins to increase less rapidly. For operation of magneto-optic display devices at 17 850 cm⁻¹ there is no advantage in using garnets in this series withx>0.8. The Faraday rotation at 17 850 cm⁻¹ increases linearly with bismuth concentration whereas the absorption coefficient increases more rapidly. The presence of Bi³⁺ increases the intensity of all Fe³⁺ pair transitions in the garnet system as a result of the increased superexchange induced by Bi³⁺. This is in keeping with the observation that the intensity of the⁶A_1g (S)→⁴T_1g (G) transition in (RE)₃Fe₅O₁₂ (RE=Er, Y, Dy, Gd, Eu) increases on traversing the above RE series as do the Curie temperatures of these iron garnets.     

Transport, optical and magnetotransport properties of hetero-epitaxial InAsxSb1−x/GaAs(x0.06) and bulk InAsxSb1−x (x0.05) crystals: experiment and theoretical analysis

We briefly review the growth and structural properties of InAs_xSb_1−x (x0.05) bulk single crystals and InAs_xSb_1−x (x0.06) epitaxial films grown on semi-insulating GaAs substrates. Temperature-dependent transport measurements on these samples are then correlated with the information obtained from structural (XRD, TEM, SEM) and optical (FTIR absorption) investigations. The temperature dependence of mobility and the Hall coefficient are theoretically modelled by exactly solving the linearized Boltzmann transport equation by inversion of the collision matrix and the relative role of various scattering mechanisms in limiting the low temperature and 300 K mobility is estimated. Finally, the first observation of Shubnikov oscillations in InAsSb is discussed.     

Thermovoltaic effect in samarium sulfide-based heterostructures

The thermovoltaic effect in samarium sulfide-based bulk heterostructures with compositions Sm_{1 − x} Eu _x S and Sm_{1 − x} Gd _x S is considered in the temperature interval 300–460 K. It is shown that this effect can be observed when an external temperature gradient is absent, i.e., when the sample temperatures near the contacts from which a generated voltage is tapped off are the same. The temperature is measured with thermocouples and also by a contactless method using a thermal imager.     

Anisotropy of plasma reflection of solid solutions (Bi2 ? xSbx)Te3 (0 < x < 1) in the Temperature Range 78–293 K

We study IR polarized reflection spectra of solid solutions (Bi_2−x Sb _x )Te₃ (0 < x < 1) in the temperature range 78–293 K and in the excitation range of plasma oscillations of free charge carriers. We reveal that, as the content of Sb₂Te₃ in the solid solution increases from 0 to 50%, the anisotropy of both plasma frequencies and magnetic susceptibility vary insignificantly. This indicates that, at 293 K, these crystals do not show significant changes in the composition of groups of free carriers near the chemical potential level. We have found that to describe the observed anisotropy of plasma frequencies, it is necessary to take into account the effect of heavy holes of the valence band. This is also confirmed by the results of the study of the temperature behavior of plasma reflection spectra, which exhibit regularities that are similar to anomalous temperature changes of the Hall coefficient. We show that a decrease in plasma frequencies with increasing temperature cannot be described by an increase in the effective mass of charge carriers in accordance with the expression m* ∼ T ^0.17, which was obtained upon interpretation of temperature dependences of a number of kinetic coefficients. We use the empirical Moss rule ɛ_∞² E _g to estimate the rate of decrease of the thermal band gap dE _g /dT = −1.6 × 10⁻⁴ eV/deg in a crystal that contains 25% Sb₂Te₃.     

EPR observations of anomalous triplet states in Ba1−x KxBiO3 and BaPbyBi1− y O3

Electron paramagnetic resonance (EPR) spectra of samples of the systems Ba_1−x K_xBiO₃ and BaPb_yBi_1−y O₃ are investigated over wide ranges of composition and temperature. Two main lines in the EPR spectrum with factors g ₁≈2.1 and g ₂≈4.2 are found for all compositions. It is shown that the observed EPR line with g ₂≈4.2 is due to oxygen ions. This probably indicates the presence of oxygen ions with different effective charges, i.e., the existence of charge density waves in the oxygen-ion sublattice in addition to charge density waves in the bismuth sublattice. Zh. éksp. Teor. Fiz. 115, 1326–1336 (April 1999)     

Resistive switching effect in thin-film heterojunctions based on electron-doped Nd2 ? xCexCuO4 ? y superconductor

Thin-film heterojunctions Nd_{2 − x} Ce _x CuO_{4 − y} /Ag were obtained. The bipolar effect of resistive switching in these heterostructures was detected and investigated. X-ray diffraction data indicate the presence of a second phase in thin films; along with the basic phase Nd_{2 − x} Ce _x CuO_{4 − y} , it affects the behavior of the interface of investigated heterojunctions and leads to an alteration of the type of conductivity. The threshold frequency of alternating voltage at which the resistive switching effect is observed in heterojunctions was detected.     

Structure, ferroelectric and piezoelectric properties of?(Bi0.98?xLa0.02Na1?x)0.5BaxTiO3 lead-free ceramics

Lead-free (Bi_0.98−x La_0.02Na_1−x )_0.5Ba _x TiO₃ ceramics have been prepared by an ordinary sintering technique and their structure, ferroelectric and piezoelectric properties have been studied. The results of X-ray diffraction show that La²⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) exists at 0.04<x<0.10. Compared with pure Bi_0.5Na_0.5TiO₃ ceramics, the (Bi_0.98−x La_0.02Na_1−x )_0.5Ba _x TiO₃ ceramics possess much smaller coercive field E _c and larger remanent polarization P _r. Because of the low E _c (3.38 kV/mm), large P _r (46.2 μC/cm²) and the formation of the MPB of rhombohedral and tetragonal phases, the piezoelectric properties of the ceramics are significantly enhanced at x=0.06: d ₃₃=181 pC/N and k _p=36.3%. The depolarization temperature T _d reaches a minimum value near the MPB. The ceramics exhibit relaxor characteristic, which is probably a result from the cation disordering in the 12-fold coordination sites. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both polar and non-polar regions at the temperatures above T _d.     

Impedance spectroscopy study of BNKLT polycrystalline ceramic

Complex impedance analysis of perovskite structured polycrystalline, [Bi_0.5(Na_1−x−y K _x Li _y )_0.5]TiO₃, at x=0.2, y=0.1 ceramic was synthesized by a mixed oxide method. The formation of single-phase material was confirmed by X-ray studies, and it was found to be rhombohedral structure at room temperature. Under scanning electron microscope, grains separated by well-defined boundaries are visible, which is in good agreement with impedance analysis. The BNKLT ceramic shows excellent piezoelectric properties and the optimum properties measured are: d ₃₃=251 pC/N, g ₃₃=24×10⁻³ mV/N, k _p =30.5% and k _t =28.1%. A complex impedance spectroscopy (CIS) study has been carried out to investigate the electrical properties. Impedance and modulus plots helped to separate the grain and grain boundary to the overall polarization or electrical behavior. CIS analysis suggests the presence of temperature-dependent relaxation process in the material. A possible hopping mechanism for electrical transport processes in the studied material is evident from the modulus analysis. The modulus mechanism indicates the non-Debye type of conductivity relaxation in the materials, which is supported by impedance data. The activation energies have been calculated from impedance (E _τ =0.58 eV) and electric modulus (E _τ =0.40 eV) studies, which suggests that the conduction is ionic in nature. The variation in width of the curves, M"/M"_maxM'/M'_{\max} and Z"/Z"_maxZ'/Z'_{\max} at FWHM, allows to conform that the relaxation process involved is of non-Debye type.     

Galvanomagnetic and thermoelectric properties of p-Bi2?xSbxTe3?ySey solid solutions at low temperatures (<220 K)

The galvanomagnetic and thermoelectric properties of p-Bi_2−x Sb_xTe_3−y Se_y solid solutions (x≤1.2, y≤0.09) are studied for various carrier concentrations. The degeneracy parameter β_d governing the scattering processes in solid solutions was calculated in terms of the many-valley energy spectrum model. The data on the degeneracy parameter and the Seebeck coefficient α were used to calculate the effective scattering parameter r _eff and the reduced Fermi level η. The parameter r _eff was found to depend on the carrier concentration in the materials studied. The temperature dependences of the effective density-of-states mass m/m ₀ and mobility μ₀ in samples with various carrier concentrations were determined. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 8, 2004, pp. 13662–1371. Original Russian Text Copyright ? 2004 by Luk’yanova, Kutasov, Popov, Konstantinov.     

Effects of ion-doping at different sites on multiferroic properties of BiFeO3 thin films

A-site Ce and B-site Zr codoped Bi_1−x Ce _x Fe_1−y Zr _y O₃ (BCFZ) thin films with different compositions were successfully prepared on the Pt/Ti/SiO₂/Si substrates by chemical solution deposition. The influence of the A-site Ce and B-site Zr codoping on the structure, surface morphology, electrical and magnetic properties of BFO films were investigated, respectively. The comparative study suggested that the A-site Ce doping with various contents have notable influences on the electrical properties of the BFO films, while the B-site Zr doping with different contents affect mainly the magnetic properties of the BFO films. Compared with the other BCFZ films studied here, the Bi_0.97Ce_0.03Fe_0.97Zr_0.03O₃ film showed the lowest dielectric loss and leakage current density, a well-squared P–E loop and fatigue-free characteristics as well as the strong magnetization.     

Effect of Sb substitution on the superconductivity of Bi2(Ca,Sr) n+1Cu n O4n+2 (n=1, 2 and 3) systems

Effect of incorporation of Sb in place of Bi in the bismuth cuprate superconductors has been examined. The nominal compositions studied are MCa_1·5Sr_1·5Cu₂O_8+δ and MCa₂Sr₂Cu₃O_10+δ, whereM=Bi_2−x Sb _x or Bi_1·5−x Sb _x Pb_0·5. Different preparative routes such as the ceramic method, the matrix route as well as the melt route were employed to prepare the materials. No indication of either Sb entering the lattice or enhancement ofT _c is noted from resistivity, magnetic susceptibility and microwave absorption measurements. Communication No. 162 from Materials Research Centre.     

Paramagnetic defects in neutron-irradiated phenakite crystals

Radiation-disordered crystals of phenakite Be₂SiO₄ (at fast neutron fluences of (0.75, 1.0, 1.2, 6.6, 8.5) × 10¹⁸ cm⁻²) have been investigated using EPR spectroscopy. It has been established that the main radiation damages predominantly occur in the silicon-oxygen sublattice of the crystal. Vacancy centers (E′ centers, g = 2.0015), defects of the displaced oxygen type, and O⁻ hole-type centers have been revealed. The paramagnetic absorption signal with the parameters g _x = 2.0218, g _y = 2.0124, and g _z = 2.0027 has been identified with the [SiO₄]³⁻ complex (a variety of O⁻ centers). The intense EPR signals with the parameters g _x = 2.0526, g _y = 2.0020, g _z = 2.0066 and g _x = 2.0290, g _y = 2.0030, g _z = 2.0099 have been assigned to two types of O₂⁻ molecular ions with different local environments. The theoretical models of radiation-induced centers have been discussed by comparing the EPR and optical spectroscopic data.     

The emission spectrum of <Emphasis Type="Italic">p</Emphasis>-AlGaAs/GaAsP/<Emphasis Type="Italic">n</Emphasis>-AlGaAs diodes under uniaxial compression

New experimental data are presented on the effects of uniaxial compression of up to 4 kbar along the [110] and [1$ \bar 1 $ \bar 1 0] crystallographic directions on the spectra of electroluminescence and the current-voltage characteristics of diodes based on n-Al _x Ga_{1 − x} As/GaAs _y P_{1 − y} /p-Al _x Ga_{1 − x} As (y = 0.84) heterostructures that were designed for injection lasers. With increasing pressure, the spectra show a shift to shorter wavelengths, reaching 25 meV at 3 kbar; the intensity increases 2–3 times as well. Numerical calculations were carried out on the band structure of the investigated heterostructures under compression along the [110] axis, which indicate the increase in the effective band gap in the quantum well (QW) GaAs _y P_{1 − y} , with a pressure coefficient of about 8.5 meV/kbar and reduction of the barrier height at the boundaries of the QW. The calculations predict the possibility that light and heavy holes crossover at pressures above 4.5–5 kbar. The increase in the effective band gap completely describes the experimental data on the shift of the electroluminescence spectra. The mixing of light and heavy holes when approaching the band crosspoint is the probable cause of an increase in the intensity of radiation under uniaxial compression.     

Paramagnetic susceptibility of Bi-Mn alloys in the liquid state

The magnetic susceptibility of Bi_{100 − x} Mn _x (x = 5, 7.5, 10, 12.5, 15, 17.5, 20, 25) alloys is experimentally studied by the Faraday method in the temperature range T = 300−1200°C and the magnetic-field range B = 0.6−1.3 T. To calculate the electronic characteristics of the Bi-Mn alloys, the experimental results are approximated by the generalized Curie-Weiss law. The calculated parameters of the electronic structure of the alloys demonstrate that manganese is present in the melt in an ionic state with an effective magnetic moment μ_eff ≈ 5μ_B, all Bi-Mn alloys have negative paramagnetic temperatures (which indicate the antiferromagnetic character of the exchange between transition 3d element atoms), and the density of states near the Fermi level n(E _F) is low. Therefore, the Fermi level is outside the d band of manganese and its position is controlled by the sp band of bismuth.     

Influence of thermal treatment in oxidizing and reducing atmospheres on the composition and properties of solid solutions based on bismuth vanadate: (Bi,La)4(V,Me)2O11?y (Me = Ga, Zr, Cu)

Ceramic solid solutions (Bi_{1 − y} La _y )₄(V_{1 − x} Me _x )₂O_{11 − z} (x, y < 0.2: Me = Zr, Ga, Fe, Cu) were prepared by solid-state reaction. It was shown that the annealing (973 K, reducing atmosphere H₂/Ar (20/80)) of the samples whose compositions belong to the stability domains of α, β, and γ′ polymorphs increases their electronic conductivity by six orders of magnitude. The samples with low concentrations of dopant cations exhibited good compositional stability and a reversible change in their structure parameters. At the same time, the solid solutions with a high concentration of lanthanum cations and/or copper cations (y, x ∼ 0.1) underwent partial decomposition.     

Simulation of temperature-dependent material parameters and device performances for GaInAsSb thermophotovoltaic cell

Temperature-dependent material parameters and device performances of Ga_xIn_1−xAs_1−ySb_y TPV cells applied in low temperature (800–1200 °C) radiators are simulated using the PC-1D. As is well known, the optimum bandgap (E_g) decreases towards lower radiator temperatures. So far, the lowest achievable E_g of Ga_xIn_1−xAs_1−ySb_y at 300 K is 0.5 eV. We mainly considering the Ga_0.8In_0.2As_0.18Sb_0.82 (E_g = 0.5 eV) TPV cell. The effects of doping concentration and recombination mechanisms of the emitter layer on photovoltaic conversion efficiencies (η_cel) are analyzed in detail, and η_cel can be improved by optimizing doping concentration and suppressing carrier recombination. The effects of GaSb window layer on η_cel are also presented. It shows the type-II energy-band alignment GaSb(window)/GaInAsSb(emitter) heterostructure affect η_cel mainly through V_oc. For the first time, the effects of operating temperatures on device performances are analyzed based on temperature-dependent material parameters, and the temperature coefficients of the device performances are presented.